Dimming in a dimmable electronic ballast for an electrodeless discharge lamp is realized by the repetition of turning the lamp on and off in the same way as, for example, the circuit described in Japanese Patent Application Publication Number 2000-353600. In such a circuit, each of lamp turn on and off periods is set to a short period of time (e.g., a few ms or less) in which brightness change by the lamp is not perceived by the human eyes. In the circuit, each lamp turn off period is set to approximately 0.5 ms.
This sort of ballast is made up of, for example, a DC power supply circuit, an inverter circuit, a resonance circuit and an induction coil. In addition, the ballast is further provided with a start circuit for sweeping up the start voltage applied across the coil in order to start an electrodeless discharge lamp successfully and stably.
In that sweep-up type ballast, the start voltage swept up during a start period becomes very high owing to the time constant of the start circuit. Because restart voltage must be raised up to voltage required to restart the lamp immediately after a short period of lamp turn off, and also the start period is longer than the restart period.
When the ballast is further provided with a ferrite core like, e.g., the system described in Japanese Patent National Publication Number P2003-515898A (WO01/041515), if ambient temperature is high, it easily saturates the ferrite core during a start period.